The commercial channel, which is replacing the series with a factual crime special next week, took a 14.1% share of viewing at 9pm-10pm.
The whole six-part series averaged 4.1 million viewers and a 17% share.
BBC One took a 22.3% share as 5.4 million viewers tuned in to see gardening expert Monty Don look into his ancestry on 'Who Do You Think You Are?'.
After ITV1 the slot's next biggest commercial terrestrial ratings went to Channel 4, which picked up 1.76 million viewers and a 7.3% share with documentary 'The Hospital'. Including Channel 4+1 ratings it was watched by 1.95 million.
Five scored an average of 1.52 million viewers for Arnold Schwarzenegger film 'Collateral Damage' between 9pm and 11.15pm.
BBC Two was the third most watched channel at 9pm-10pm with 2.84 million viewers for 'Dragon's Den', including 50,000 seeing it at the same time on BBC HD.
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